MasterPiece Arms (MPA) is now selling a side folding stock kit for their firearms. The kit comes with a stock, but will also accept any standard AR-15/M-4 stock as well via an included adaptor.
The kits come in two sizes depending on which of the MPA guns you are looking to outfit. The stock locks into place at both the open and closed positions.
MPA is very clear in the literature that attaching this to a pistol makes it into an NFA item: a short barrel rifle (SBR). As silly as it may sound putting a stock on a pistol is a serious crime in the United States.
The kit retails for $129.99.